Cast cylindrical polyvinyl alcohol brushes are conventionally used in automatic cleaning systems to provide a post CMP (Chemical Mechanical Planarization) process to effectively clean surfaces of substrates such as semiconductor wafers or other disc-shaped substrates. Cylindrical polyvinyl alcohol brushes are also used in cleaning systems to clean and dry glass and other non-disc-shaped substrates in flat panel display manufacture, glass production, and printed circuit board assembly. Cylindrical brushes may have a length as short as 50 millimeters or as long as 10 meters, for example. The cylindrical brushes extend completely across the substrate being treated, thereby contacting the substrate an entire distance across its diameter.
The cylindrical brushes are located on and driven by a central brush core about a central longitudinal axis in the cleaning process. The cylindrical brushes have a constant cross-sectional diameter when taken perpendicular to the central axis along the length of the brush. An accurate and stable connection between the cylindrical brush and the central brush core is desirable. The cylindrical brushes may have nodules on their outer surface to help clean the substrate.
The cylindrical brushes are expected to accurately rotate on their axis and provide a generally cylindrical surface with a generally consistent nodule pressure pattern. In some cases, the cylindrical brush is formed around the central brush core. For example, the brush core may be placed in a mold and a mixture of chemicals, such as polyvinyl alcohol, is injected into the mold to form the cylindrical brush around the central brush core.
When a rotating cylindrical brush engages a rotating circular substrate, such as a semiconductor wafer, slippage between the cylindrical brush and the circular substrate may occur due to differences between linear speeds of points on the cylindrical brush and the circular substrate. Cylindrical brushes rotate at a generally constant rotational speed, and since the brushes are cylindrically shaped, the linear speed of any point on an outer surface of the cylindrical brush is generally constant. As defined herein, the linear speed of a point on a surface or substrate is the speed of that point in a direction which is tangential to a rotational direction of that point. However, as the circular substrate rotates at a generally constant rotational speed, the linear speed of points near the outer edge of the circular substrate is much greater than the linear speed of points near or at the center of the circular substrate. As a result, it becomes impossible to match the linear speed of all the points on the outer surface of the cylindrical brush with adjacent points engaged on the circular substrate, resulting in some slippage between the cylindrical brush and the circular substrate at an engagement area along which the cylindrical brush engages the circular substrate. This slippage becomes a greater issue for larger sized substrates.
As a result, it would be desirable to have a brush in which the linear speed of all the points on the outer surface of the brush is nearly matched with the linear speed of adjacent points engaged on the circular substrate, resulting in less slippage between the cylindrical brush and the circular substrate at the engagement area along which the cylindrical brush engages the circular substrate.